The present invention generally relates to a photography and more particularly, to a photographic camera by which an object to be photographed can be taken in a pseudo focal length photographing mode as well as in a real focal length photographing mode.
In taking a photograph, it is especially important to determine a composition of an image to be represented on a print. Then, there has been known a method for changing the composition of an area of an object to be recorded on a frame of a film by altering the distance between a camera and the object, or by altering angle of view of a photographing lens with zooming operation of a zoom lens or with interchanging the objective lens mounted on the camera to that having different focal length. On the other hand, the other method has been known in which only a desired area of an image recorded on a frame is printed with enlargement. This method is called "trimming" in the field of photography. Thus, a print having a desired composition can be reproduced by using either of the above two methods.
One of the methods related to the latter for printing the negative through the trimming process is disclosed in the Japanese Patent Laid-open Application (Tokkaisho) No. 54-26721 in which the photographic camera is provided with an optical finder system capable of selectively changing the field of view. By the aforementioned optical finder system, not only a scope of the object to be photographed is selected in the photographic camera, but also an information for trimming is recorded on a film loaded in the camera in accordance with the selection determined by using the optical finder system. Then, in case of the printing, a selected portion of the image is printed in accordance with the trimming information with an enlargement magnification different from that in the usual case, thus resulting in that it is capable of obtaining the print which looks like a print obtained from the frame which has been taken through a telephoto lens having a focal length longer than that of the objective lens actually mounted on the camera.
On the other hand, when a comparison is made between the photograph taken in the pseudo focal length photographing mode and another photograph taken, with respect to the same object as that of the former, through the lens having the same real focal length as is purposely set by the pseudo focal length photographing mode, it is readily appreciated that the photograph of the former has a depth-of-field deeper than that of the latter.
Hereupon, the depth-of-field a1 of the lens having the focal length of f1 is obtained as follows. ##EQU1## where the distance up to the object is U on the assumption that U&gt;&gt;f, an F-number of the lens is F and the diameter of a permissible circle of confusion is .delta..
Likewise, the depth-of-field a2 of the lens having the focal length of f2 can be obtained by the following equation. ##EQU2##
Meanwhile, suppose that an image of a frame photographed with using the lens having the focal length of f1 is printed with enlarging so as to be equivalent to a print enlarged and photographed with using the lens having the focal length of f2, the diameter of the permissible circle of confusion on the print should be limited to .delta. with respect to the frame photographed with using the lens having the focal length of f1. Therefore, the diameter of the permissible circle of confusion on the image to be recorded on the film should be .delta..times.f1/f2.
The depth-of-field a3 in this case is expressed as follows. ##EQU3##
Accordingly, when the comparison is made between the depth-of-field a3 in the case where the photograph taken through the lens having the focal length of f1 is enlarged as described above and the depth-of-field a2 of the photograph taken through the lens having the focal length of f2, the following equation can be obtained. EQU a3.apprxeq.a2 (f2/f1)
As a result, the photograph obtained through the trimming process has f2/f1 times deeper in the depth-of-field than that of the photograph taken through the lens having the focal length of f2, and since this proportion of depth-of-field corresponds to a ratio of enlargement in printing between the two prints.
As described above, since the photograph obtained through the trimming process has the deeper depth-of-field, both of a character and a background behind it are clearly photographed as, for example, in a snapshot taken during travelling. Although the photograph of this kind is preferable as a documentary photograph, when it is requested to obtain the photograph in which the character is caused to be in focus with the background behind it being blurred, the desired photograph can be hardly taken in the pseudo focal length photographing mode.
Moreover, since a frame adapted to be used in the pseudo focal length photographing mode by the above described camera is inevitably printed in an elongated form by an enlargement magnification larger than that in the real focal length photographing mode, there has been such a drawback that particles on the film tend to become conspicuous.